Coating of cobalt alloys



p 9 p. H. MAXWELL ETAL 3,343,982

' COATING OF COBALT ALLOYS Filed Oct. 21, 1964 INVENTORS DOUGLAS H.MAXWELL gQANK SUYAMA United States Patent 3,343,982 COATING OF COBALTALLOYS Douglas H. Maxwell, North Palm Beach, and Frank Suyama, West PalmBeach, Fla., assignors to United Aircraft Corporation, East Hartford,Conn., a corporation of Delaware Filed Oct. 21, 1964, Ser. No. 405,53211 Claims. (Cl. 117-1072) This invention relates to the treatment ofhigh cobalt alloys to render them resistant to erosion due to oxidationat elevated temperatures.

Gas turbine blades such as rotor blades and stator vanes require the useof metal alloys which are dimensionally stable for long periods of timeat elevated temperatures. Many cobalt alloys have been found to possesssatisfactory dimensional stability at normal operating temperatures.However, at temperatures as high as 2100 F., many of the conventionalcobalt alloys show a tendency toward excessive oxidation and erosion ofthe oxidation product.

It has been proposed heretofore to coat various high temperature alloyswith chromium or aluminum in order to increase their oxidationresistance. However, cobalt alloys coated by conventional means stillshow a significant decrease in weight i.e., erosion due to oxidation,when held at 2100 F., for 50 to 100 hours. It has now been found thatsuperior oxidation resistance can be achieved if high cobalt alloys arediffusion coated with a mixture of chromium and an aluminum-magnesiumalloy containing about 15% magnesium. The thickness of the diffusioncoating is not very great, being on the order of from about 0.001" to0.005. Such coated high cobalt alloys are resistant to erosion due tooxidation for period of 100 hours or more at 2100 F.

The pack diffusion method of applying the diffusion coating is employedin this invention. Generally, in the process of this invention, theshaped alloy to be coated is embedded in a retort or other suitablecontainer containing a predetermined mixture of granular chromium metal,granular aluminum-magnesium alloy, a chromium halide, preferably chromicchloride, and an iodine source such as elemental iodine or ammoniumiodide. The retort or other container is treated to remove air, sealedand sintered in a hydrogen or other inert gas atmosphere for the timeand temperature, generally above 1800 F., sufiicient to obtain thedesired coating thickness and degree of diffusion. Cobalt alloys to betreated in accordance with this invention generally contain at leastabout 35% of cobalt. Preferably the alloy contains at least about 50%cobalt and about 15% to 27% by weight of chromium. Excellent results areobtained when such cobalt-chromium alloys also contain from 0% to 12% ofnickel, from to 12% of tungsten, from 0% to 1% of titanium, from 0.4% to1.2% of carbon, and from 0.05% to 2.5% of zirconium. In some instances,the presence of other trace materials is desirable including up totantalum, up to 3 columbium, up to 0.01% boron, up to 1.5% iron, up to0.2% manganese, and up to 0.2% silicon. Traces of such impurities assulfur, phosphorus and copper and the like in amounts which do notdetract from the thermal resistance of the basic cobalt alloy can alsobe present.

A preferred alloy for treatment in accordance with this invention is acommercial alloy known as SM 302 which has the following nominalchemical analysis:

Percent by Weight Chromium 21.5

Tungsten 10.0 Tantalum 9.0

Patented Sept. 26, 1967 Percent by weight Zirconium 0.25

Iron 1.0 Nickel max 1.5

Boron 0.01

Carbon 0.86

Cobalt, remainder.

Alloy WI 52 is a cobalt base alloy having relatively large amounts ofchromium and tungsten in its composition which is specified as follows:

Cobalt, remainder.

Alloy X 40 is a cobalt base alloy having the following specificationanalysis:

Carbon 0.45-0.55 Manganese max 1.0 Silicon max 1.0 Phosphorus max 0.04Sulfur max 0.04 Chromium 24.526.5 Nickel 9.511.5 Tungsten 7.08.0 Ironmax 2.0

Cobalt, remainder.

The granular composition making up the pack would generally contain fromabout 0.5% to 2% by weight of chromium metal, from 1.5% to 3% by weightof the aluminum magnesium alloy containing from 10% to 20% magnesium,from 0.005 to 0.008 by weight of a chromic halide, preferably chromicchloride and from 0.001%. to .005 by weight of either elemental iodineor ammonium iodide and from 95% to 98% by weight of an inert carriermaterial. The preferred inert carrier material is activated alumina,i.e., aluminum oxide, although other materials such as baked kaolin andmagnesium oxide can also be employed.

The preferred composition contains about 1% by weight of chromium, about2% by weight of an aluminummagnesium alloy, containing 15% by weight ofmagnesium, about 0.005% by weight of chromic chloride, about 0.001% byweight of either elemental iodine or ammonium iodide and about 97% byweight of activated aluminum oxide.

The granular material should be finely divided and will be sufficientlyfine to pass through a 50 mesh sieve and preferably through a meshsieve.

FIGURE 1 represents a suitable container for use in carrying out themethod of this invention. This container consists of an inner retort 1having a cover 2 both fabricated of any substance capable ofwithstanding the temperatures of operation. Deposited Within thecontainer is the granulated packing material 3 having been thecomposition described above andembedded within the packing material areone or more shaped cobalt alloy particles 4 to be coated. A glass ring 5surroundsthe inner retort at its contact point with cover 2.

In operation, the alloy to be coated is first cleaned by sandblastingand is then subsequently heat treated in a hydrogen atmosphere to removeany traces of an oxide layer. The treated alloy is then embedded in thepowder mixtures within container 1. The container is closed and placedin a suitable heating device such as a mufiie furnace equipped with agas-tight door and containing gas inlet and outlet ports. The heatingdevice is purged with an inert gas such as helium or ar on to remove theair, and thereupon the protective gas to be employed in the process,either hydrogen or an inert gas such as helium or argon is supplied.Hydrogen is preferred. The temperature within the heating device israised to a temperature of from 300 F. to 500 F., preferably about 400F. and maintained for a short time to remove any moisture containedwithin the system. The temperature is then raised slowly to just belowthe melting point of the glass 5 to allow the system to come toequilibrium temperature and to allow any remaining air or other gases toleave the retort. The temperature is then raised to above the meltingpoint of the glass ring 5, thereby causing the glass to melt and form aliquid seal isolating completely the contents of the container from theoutside environment. Thereupon the heating device is allowed to come tothe diffusion temperature, about 1800 F. to 2200 F. and maintained therefor the desired time, generally from 8 to 16 hours. The particular timeand temperature employed will, of course, vary within the suggestedrange depending upon the exact composition of the alloy and the desiredamount of coating to be applied.

The following example represents the preferred mode of carrying out theinvention.

Two samples of SM 302 alloy, a high cobalt alloy described above, weresand blasted and heated in a hydrogen atmosphere at a temperature of2000-2100 F. for two to four hours to remove any surface oxide coating.The alloy articles were then embedded in 2000 grams of a granular powdermixture within a retort. This granular powder contained 97% by weight of100 mesh activated alumina, 2% by weight of a 100-250 meshaluminummagnesium alloy containing 85% aluminum and 15% magnesium, 1% ofa 100 mesh chromium metal, 0.005% of chromic chloride and 0.001% ofelemental iodine.

The retort employed was that illustrated in FIGURE 1. After the retortwas closed with the top and the glass ring, it was placed in an ovenequipped with gas inlet and outlet ports and purged with argon to removethe air and thereupon hydrogen gas was admitted and the furnace was heldat a temperature of 400 F. for 30 minutes to remove any moisturecontained in the mix ture. The temperature of the furnace was thenraised at the rate of 200 F. per hour to about 1200 F., to allow thesystem to come to equilibrium temperature and to allow any remaining airto escape from the retort. T hereupon the temperature was raised to 1350F., 50 above the melting point of the glass and maintained for 30minutes while the glass melted and formed a liquid seal. Thereupon thetemperature was raised to 2050 F. and maintained for 16 hours. At theconclusion of this period, the furnace was allowed to cool, the glassseal was broken and the diffusion coated alloy removed. Upon testing,none of the samples showed any weight loss after being heated for 100hours at 2100 F., indicating that the coating method of this inventionis well suited for use in protecting gas turbine blades from erosion dueto oxidation. One of the samples showed a negligible weight increase atthe end of 100 hours of about 0.4%, while the other sample showed anequally negligible weight increase of about 1.6%. This contrasts with anuncoated alloy which would show a weight loss of 12% or more when heatedfor even as much as 60 or 70 hours at 2100 F. v

Similar results are obtained when other high cobalt alloys are employed.

We claim:

1. The method of forming a protective coating on a oobalt alloycontaining at least about 35% of cobalt, which renders the alloyresistant to oxidation at elevated temperatures, comprising embeddingthe alloy to be coated in a granular mixture of from 0.5% to 2% byweight of chromium metal, from 1.5% to 3% by weight of analuminum-magnesium alloy containing from 10% to 20% magnesium, from0.005% to 0.008% by weight of chromic chloride and from 0.001% to 0.005%by weight of a substance selected from the group consisting of elementaliodine and ammonium iodide, and from to 98% by weight of an inertcarrier material, and heating the embedded alloy, in a protectiveatmosphere, at a temperature, at least about 1800 F., and a timesuflicient to obtain the desired coating thickness.

2. The method of claim 1 wherein the inert carrier material is activatedaluminum oxide.

3. The method of claim 2 wherein the granular mixture is of a particlesize small enough to pass through a 50 mesh sieve.

4. The method of claim 3 wherein the cobalt alloy contains from 15 to27% by weight of chromium, from 5% to 12% of tungsten, from 0.4% to 1.2%of carbon, from 0.05% to 2.5% of zirconium, up to 12% nickel and up to1% titanium.

5. The method of claim 4 wherein the protective atmosphere is a hydrogenatmosphere.

6. The method of claim 3 wherein the protective atmosphere is a hydrogenatmosphere.

7. The method of claim 3 wherein the cobalt alloy is embedded in thegranular mixture within a container, and the temperature is slowlyelevated to permit the escape of extraneous gases and then the meltingof a glass ring to form a liquid glass seal closing the container to thepassage of gases.

8. The method of forming a protective coatin on a cobalt alloycontaining at least about 50% by weight of cobalt and from about 15% to27% by weight of chromium, thereby rendering the alloy more resistant tooxidation at elevated temperatures, comprising embedding the alloy to becoated on a finely divided granular mixture containing about 1% byweight of chromium, about 2% by weight of an aluminum-magnesium alloycontaining 15 by weight of magnesium, about 0.005 by weight of chromicchloride, about 0.001% by weight of a substance selected from the groupconsisting of elemental iodine and ammonium iodide and about 97% byweight of activated aluminum oxide, and heating the embedded alloy, in ahydrogen atmosphere, at a temperature, at least about 1800 F., at atime, sutficient to obtain the desired coating thickness.

9. The method of claim 8 wherein the cobalt alloy is embedded in thegranular mixture within a container and the temperature is slowlyelevated to permit the escape of extraneous gases and then the meltingof a glass ring to form a liquid glass seal closing the container to thepassage of gases.

10. The method of claim 9 wherein the cobalt alloy contains from 15 to27% by weight of chromium, from 5% to 12% of tungsten, from 0.4% to 1.2%of carbon, from 0.05% to 2.5% of zirconium, up to 12% nickel and up to1% titanium.

11. The method of claim 9 wherein the cobalt alloy has the followingapproximate chemical composition Percent Chromium 21.5

Tungsten 10 Zirconium .25

Tantalum 9 Iron 1 Nickel up to 1.5 Boron 0.01

Carbon 0.9

Cobalt, remainder.

(References on following page) 5 6 References Cited 3,163,553 12/1964Commanday 117107.2 3,257,227 6/1966 Seelig 117 -107.'2 UNITED STATESPATENTS 3,257,230 6/1966 Wachtell 117-1072 3,096,160 7/1963 Puyear117-107.2 3,096,205 7/1963 De Guisto 117 107.2 5 ALFRED LEAVITT,Pl'lmary Exammer- 3,157,532 11/1964 Galmiche 117 107.2

A. GOLIAN, Examiner.

1. THE METHOD OF FORMING A PROTECTIVE COATING ON A COBALT ALLOYCONTAINING AT LEAST ABOUT 36% OF COBALT, WHICH RENDERS THE ALLOYRESISTANT TO OXIDATION AT ELEVATED TEMPERATURES, COMPRISING EMBEDDINGTHE ALLOY TO BE COATED IN A GRANULAR MIXTURE OF FROM 0.5% TO 2% BYWEIGHT OF CHROMIUM METAL, FROM 1.5% TO 3% BY WEIGHT OF ANALUMINUM-MAGNESIUM ALLOY CONTAINING FROM 10% TO 20% MAGNESIUM, FROM0.005% TO 0.008% BY WEIGHT OF CHROMIC CHLORIDE AND FROM 0.001% TO 0.005%BY WEIGHT OF A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF ELEMENTALIODINE AND AMMONIUM IODIDE, AND FROM 95% TO 98% BY WEIGHT OF AN INERTCARRIER MATERIAL, AND HEATING THE EMBEDDED ALLOY, IN A PROTECTIVEATMOSPHERE, AT A TEMPERATURE, AT LEAST ABOUT 1800*F., AND A TIMESUFFICIENT TO OBTAIN THE DESIRED COATING THICKNESS.